Voltage-sensitive switch



P 1970 E. N. URFER ETAL 3,531,694

VOLTAGE-SENSITIVE SWITCH Filed July 18, 1968 United States Patent O3,531,694 VOLTAGE-SENSITIVE SWITCH Ernest N. Urfer, North Adams, John.I. Randall, Jr., Williamstown, and Henry F. Puppolo, North Adams,Mass., assiguors to Sprague Electric Company, North Adams, Mass., acorporation of Massachusetts Filed July 18, 1968, Ser. No. 745,879 Int.Cl. HOlg 3/075 US. Cl. 317-230 Claims ABSTRACT OF THE DISCLOSURE Acounterelectrode overlies the anodic oxide layer formed on the surfaceof an aluminum electrode. The aluminum electrode contains at least oneisolated, visible iron-containing impurity site. The voltage-sensitiveswitch is formed by chemically or electro-chemically cleaning andpolishing an aluminum electrode, treating the cleaned and polishedelectrode with a hydroxyl-containing polar compound, preferably water,for a period suflicient to make the iron-containing impurity sitelatently visible, anodizing the treated surface in chromic acid to forman anodic oxide layer which reveals said iron-containing impurity site,and applying a counterelectrode. The method can also be employed todetermine the presence or absence of ironcontaining impurity sites inaluminum.

BACKGROUND OF THE INVENTION The present invention is concerned with avoltage-sensitive switch and a method for forming the same. Theinvention is also concerned with a method for determining the presenceor absence of iron-containing impurity sites in aluminum.

The voltage-sensitive switch is a thin film solid state device which isdesigned to short circuit When a pulse voltage of predeterminedamplitude is applied across its terminals. Switches of this type aredescribed in co-pending US. patent application S.N. 326,152, filed Nov.26, 1963 in the names of Henry F. Puppolo et al., now Pat. 3,412,220.The switch defined in this co-pending patent application is thecombination of (1) a fully annealed aluminum foil having at least onebright, smooth electropolished surface; (2) a first layer on saidsurface anodically formed from a chromic acid electrolyte, the resultingoxide being relatively porous; (3) a second layer on the first formedlayer, anodically formed from a phosphate electrolyte, the resultingoxide being relatively dense; and (4) a conductive film of finelydivided metal as the counterelectrode of the device. It is extremelyimportant that the closure of a voltage-sensitive switch be reliablewithin a very narrow range.

It is an object of the invention to present a novel voltagesensitiveswitch of high reliability;

It is another object of the invention to present a new process forpreparing a voltage-sensitive switch; and

It is yet another object of the invention to present a process fordetermining the presence or absence of ironcontaining impurity sites inaluminum.

SUMMARY OF THE INVENTION It has been determined that the closure of avoltagesensitive switch results from oxide film breakdown atpreferential sites. These sites can now be examined prior to breakdownby viewing the anodized aluminum under low power magnification. It hasbeen discovered that if an aluminum electrode having iron-containingimpurity sites is cleaned in a specific manner and then the cleanedsurface treated with a hydroxyl-containing polar compound, preferablywater, for a certain minimum period, then,

after anodization of the aluminum in a specific electrolyte, theiron-containing impurity sites are plainly visible.

The process for forming the voltage-sensitive switch can be summarizedas follows: Chemically or electrochemically polishing at least onesurface of an aluminum electrode so as to leave the surface clean andsmooth with only a surface oxide, the thickness of which is of the orderof naturally occurring oxide, said surface having at least oneiron-containing impurity site. Treating the cleaned surface with ahydroxyl-containing polar compound, preferably water, for a periodsutficient to make the ironcontaining impurity site latently visible.Anodizing the treated surface in an aqueous chromic acid solution toform an anodic oxide layer and make visible said ironcontaining impuritysite. The anodized surface is then rinsed to remove chromic acid and acounterelectrode is applied to the anodic oxide. In a preferredembodiment a second anodization in a phosphate electrolyte is performedto form a second dielectric which heals imperfections and partiallyfills the pores and voids of the first dielectric oxide.

The process can also be employed to determine the presence or absence ofiron-containing impurity sites in aluminum by taking a sample ofaluminum and performing the foregoing sequence of steps up through thestep of rinsing to remove chromic acid. At this point visual inspectionof the anodic oxide surface will reveal the presence or absence ofiron-containing impurity sites. By this technique one can determinewhether or not the aluminum represents raw material of minimallyacceptable quality for the preparation of voltage-sensitive switches. Ifthe process does not reveal at least one iron-containing impurity siteper switch area, this aluminum should not be used in a voltage-sensitiveswitch. If the aluminum is iron-free, or substantially so, the aluminumwill represent good raw material for aluminum capacitor manu facture.

The device of the present invention comprises an aluminum electrodehaving at least one comparatively clean surface; an aluminum oxide layeranodically formed on said surface, said oxide revealing at least oneisolated iron-containing impurity site; and a counterelectrode appliedto said dielectric oxide. In a p eferred embodiment of thevoltage-sensitive switch of t e present invention, a second aluminumoxide heals imperfections and partially fills the pores of the firstaluminum oxide. The device of the present invention can be prepared fromfully annealed aluminum foil or from hard aluminum foil and a preferredcounterelectrode for the present invention is silver.

BRIEF DESCRIPTION OF THE DRAWING The drawing represents a side view insection of part of a voltage-sensitive switch within the scope of thepresent invention.

The drawing shows a cross-sectional view of part of a voltage-sensitiveswitch 10. An aluminum electrode 12 has formed thereon an aluminum oxidedielectric 14. Said aluminum oxide has voids or pores 16 therein. Ametal counterelectrode 18 is applied to the aluminum oxide di electric.Located in the surface of the aluminum electrode is an iron-containingimpurity site 20. The iron-containing impurity site is located in thecenter of a crater or depression 22. The region 24 bounded by the dottedlines repre sents the short or closure region of the device.

DETAILED DESCRIPTION OF THE INVENTION As previously indicated, it hasbeen determined that the closure or shorting of a voltage-sensitiveswitch results from oxide film breakdown at preferential sites. Bycarefully cleaning and polishing the surface of an aluminum electrode,followed by treating the cleaned and polished surface with ahydroxyl-containing polar compound, preferably water, and then byanodizing the aluminum in a specific electrolyte, the iron-containingimpurity sites are made plainly visible. Electron probe micro-analysisof the breakdown sites has revealed a concentration of metallicimpurities at these locations. The impurity sites were determined tocontain iron with considerably lesser amounts of nickel and chromium. Inorder to prepare a reliable voltage-sensitive switch it is necessarythat the aluminum electrode contains at least one iron-containingimpurity site. The electrode must be cleaned with a reagent whichremoves all surface reaction product from the surface of the aluminum orat most which leaves a thin film somewhat akin to the comparatively thinnatural oxide which forms on aluminum. This thin film can, for thepurposes of the present invention, be ignored since it does notcontribute materially to the resistance to physical breakdown of theunit. One such cleaning reagent is a perchloric acidacetic anhydridesolution. A chemical polishing solution which was not effective inproperly cleaning an aluminum electrode is a phosphoric acid-nitric acidsolution containing a small amount of cupric nitrate. Switches preparedfrom aluminum electrodes polished with this ineffective solution showedextremely high closures and no defect or impurity sites were observedafter anodization of the aluminum electrodes.

After the aluminum electrode is properly cleaned the surface must betreated with a hydroxyl-containing polar compound, preferably water, fora sufficient length of time so that the iron-containing impurity sitesin the aluminum will be rendered latently visible. The term latentlyvisible means that on anodization of the aluminum electrode in theproper anodization electrolyte, the iron-containing impurity sites willbe rendered plainly visible under low magnification. Water temperatureand water flow will affect the time required to render the ironcontaining impurity sites latently visible. For example, using runningdistilled water at about 19 C., the impurity sites will become latentlyvisible after about 1 minute. However, using static water, the time willbe considerably longer. If the water temperature approaches 100 C.,there is the likelihood of forming a hydrated film on the electrodewhich will result in the voltage-sensitive switch having extremely highand erratic closure voltages.

Investigation also has determined that only certain anodizationelectrolytes will properly develop or make plainly visible theiron-containing impurity sites. The anodization electrolyte should be ofthe type which produces a socalled porous electrolyte. Electrolytes ofthis type are represented by a chromic acid electrolyte or an oxalicacid electrolyte. An electrolyte such as ammonium dihydrogen phosphatedoes not reveal the presence of the iron-containing impurity sites andthus is not an effective anodization electrolyte for the initial anodicoxide layer of the voltage-sensitive switch of the present invention.While ammonium dihydrogen phosphate is not an effective electrolyte forthe formation of the initial anodic oxide, it is a preferred electrolyteto be employed for the purpose of healing imperfections and partiallyfilling the voids or pores which naturally occur in an oxide formed withan electrolyte such as chromic acid.

Tthe counterelectrode of the voltage-sensitive switch can be any one ofa number of prior art electrodes, however, a preferred counterelectrodeis silver.

The starting material for the switch of the present invention asindicated must have at least one iron-containing impurity site perswitch area. Commercially available 99.88 percent, 99.99 percent or99.999 percent aluminum can be employed. While the aluminum either canbe fully annealed or hard aluminum, it is preferred to use fullyannealed material in order to avoid accidental cracking of the anodicoxide which is more common when hard foil is employed.

The following is a specific example of the preparation of avoltage-sensitive switch within the scope of the present invention:

Example I A length of 3 mil aluminum foil of 99.99% purity and known tocontain iron-containing impurity sites is electropolished in thesolution made by mixing 138 milliliters of 60% perchloric acid (inwater) with 483 milliliters of acetic anhydride. The foil is placed inthe solution with one face of the foil opposite an aluminum cathode alsoimmersed in this solution. An anodic current density of 10 milliamperesper square centimeter of said foil surface is passed through the systemfor 2 /2 minutes, the electrolyte being kept at about 23 C. and activelyagitated. The foil is now much brighter in appearance. The polishedsample is treated in running distilled water (flow rate 3 liters perminute) at 19 C. for a period of about 6 /2 minutes. The treated sampleis anodized in a 1 /2% chromic acid electrolyte at F. at 1 ma./cm. forabout 10 minutes. The anodized sample is then rinsed to remove thechromic acid. At this point, using a magnification of about 68 times,the iron-containing impurity sites are plainly visible as circular areashaving a central dot which has been established as the site of the iron.The unit is then dried and a conductive film applied, using anelectrically conductive silver paint such as that described in theBureau of Standards circular 46 8, pages 5-7. Using a ramp circuit i.e.a circuit generating a linearly increasing voltage, the unit of theexample was eventually subjected to a voltage which caused closure orshorting of the electrodes. Photomicrographic examination of the unitshowed that the shorting of this device occurred at at least one of theiron-containing impurity sites.

Example II In a preferred embodiment of the present invention theprocess of Example I is repeated but after anodization in chromic acidfollowed by rinsing, a second anodization step is carried out in a 0.01%ammonium dihydrogen phosphate electrolyte in order to heal imperfectionsand partially fill the pores which tend to naturally occur in thechromic acid-formed aluminum oxide. A preferred percentage range for thephosphate electrolyte is from 0.005 to 1.0%. The unit is then dried anda counterelectrode is added as in the foregoing example.

Units formed by the process of the present invention will reliablybreakdown within an extremely narrow voltage range. No difliculty isexperienced in preparing units which will reliably short at anypredetermining voltage within the range of about 10 to 100 volts.

The chemical or electrochemical polishing agents contemplated hereinmust be capable of cleaning and smoothing the surface of the aluminumand leave no more than a surface oxide of not substantially greaterthickness than that of the naturally occurring oxide. While thepreferred polishing agent is a perchloric acid-acetic anhydridesolution, other agents which will clean and polish aluminum tosubstantially the same degree can also be employed. In this respectreference is made to The Electrolytic and Chemical Polishing of Metalsby W. J. McG. Tegart, published in 1959 by Pergamon Press.

In addition to water, which is preferred, other hydroxyl-containingpolar compounds which may be employed herein are methyl alcohol, ethylalcohol, alone or in combination or in combination with water.

The chromic acid anodization can be carried out 111 differentconcentrations of the acid. Chromic acid can be replaced by otherelectrolytes, such as oxalic acid, known to produce porous oxide filmssimilar to that produced with chromic acid. The ammonium dihydrogenphosphate in the second anodization can be replaced by otherelectrolytes known to produce compact oxide films, such as diammoniumhydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogenphosphate.

The preferred counterelectrode metal is silver, however,

other metals and conductive films may substitute therefor.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention it is to be understood that the invention is notlimited to said details except as set forth in the appended claims.

What is claimed is:

1. A method for determining the presence or absence of iron-containingimpurity sites in aluminum comprismg:

(a) chemically or electrochemical polishing at least one surface of analuminum sample so as to leave the surface clean and smooth with asurface oxide film of not substantially greater thickness than that of anaturally occurring aluminum oxide film produced under normalatmospheric conditions;

(b) treating the cleaned surface with a hydroxyl-containing polarcompound to render any iron-containing impurity site latently visible;

(c) anodizing the treated surface in a porous oxideforming electrolyteto form an aluminum oxide layer and make visible any iron-containingimpurity sites; and

(d) rinsing the anodized surface to remove electrolyte thereby exposingany iron-containing impurity sites.

2. The method of claim 1 wherein the electrochemical polishing iseffected in a perchloric acid-acetic anhydride solution; thehydroxyl-containing polar compound is water; and the porousoxide-forming electrolyte is an aqueous chromic acid solution.

3. The method of claim 2 wherein said aluminum sample is aluminum foil;the electropolishing solution contains about 1 part 60% perchloric acidto from 2-4 parts acetic anhydride; the water-treating period is atleast about one minute; and anodization is effected in a 0.01 to 1%chromic acid solution.

4. The method of claim 3 wherein said aluminum is fully annealed.

5. The method of claim 3 wherein said aluminum is hard foil.

6. A method for preparing a voltage-sensitive switch comprising thesteps:

(a) chemically or electrochemically polishing at least one surface of analuminum electrode, so as to leave the surface clean and smooth with asurface oxide film of not substantially greater thickness than that of anaturally occurring aluminum oxide film produced under normalatmospheric conditions, said surface having at least one iron-containingimpurity site;

(b) treating the clean surface with a hydroxyl-containing polar compoundto render said iron-containing impurity site latently visible;

(c) anodizing the treated surface in a porous oxideforming electrolyteto form an aluminum oxide layer and make visible any iron-containingimpurity sites;

(d) cleaning the applied surface to remove electrolyte;

and

(e) applying an electrode to said anodic oxide.

7. The method of claim 6 wherein the electrochemical polishing iseffected in a perchloric acid-acetic anhydride solution; thehydroxyl-containing polar compound is water; and the porousoxide-forming electrolyte is an aqueous chromic acid solution.

8. The method of claim 7 wherein said aluminum electrode is aluminumfoil; the electropolishing solution contains about 1 part perchloricacid to from about 2+4 parts acetic anhydride; the water-treating periodis at least about one minute; and anodization is effected in a 0.01 to10% chromic acid solution.

9. The method of claim 8 wherein after the chromic acid anodization,performing a second anodization in a phosphate electrolyte to form asecond, compact dielectric which heals imperfections and partially fillsthe pores and voids in the chromic acid formed oxide.

10. The method of claim 9 wherein the second anodization is effected ina 0.005 to 1.0% aqueous electrolyte selected from the group consistingof ammonium dihydrogen phosphate, diammonium hydrogen, phosphate, sodiumdihydrogen phosphate and disodium hydrogen phosphate.

11. A voltage-sensitive switch comprising an aluminum electrode havingat least one comparatively clean surface; a porous aluminum oxide layeranodically formed on said surface, at least one isolated iron-containingimpurity site and said oxide layer plainly revealing said at least oneisolated iron-containing impurity site; and a counterelectrode appliedto said oxide.

12. The switch of claim 11 wherein said surface is an electropolishedsurface.

13. The switch of claim 12 wherein a second, compact aluminum oxideheals imperfections and partially fills the pores and voids of saidporous aluminum oxide.

14. The switch of claim 13 wherein said electrode is fully annealedaluminum foil and said counterelectrode is silver.

15. The switch of claim 13 wherein said electrode is hard aluminum foiland said counterelectrode is silver.

References Cited UNITED STATES PATENTS 1,816,875 4/1931 Sicgmund 3l7230X 2,094,048 9/ 1937 Siegel 317230 X 2,408,910 10/1946 Burnham 3l7230 XJAMES D. KALLAM, Primary Examiner US. Cl. X.R.

